This invention relates generally to an electrophotographic printing system and, more particularly, to a folded optical system which scans a document and projects the scanned image in a precession mode onto a moving flat photoreceptor.
Precession scan systems which expose images on a flat photoreceptor in a direction opposite the photoreceptor movement are known in the art. The advantages of such systems are also well known. U.S. Pat. No. 4,336,995 discloses the velocity relationships which must be present between the document scanning optical components and the photoreceptor. U.S. Pat. No. 4,351,605 discloses a precession system operating under the dual conditions of document scan or continuous velocity transport. U.S. Pat. No. 4,362,382 discloses a specific scan system utilizing a moving corner mirror assembly between the lens and the photoreceptor to accomplish the image precession. U.S. Pat. No. 4,484,810 discloses a hybrid precession scanning system where the platen, scan elements and lens are all moved in specific relationships with each other and with the photoreceptor.
In U.S. patent application No. 642,272, filed on Aug. 20, 1984, now U.S. Pat. No. 4,585,331, and assigned to the same assignee as the present invention, there is disclosed a precession scanning system in which the mirror clusters comprising the scan elements are each driven by an independently controlled linear drive motor.
The advantages of utilizing a precession system are well understood. In any scanning system, the scan components (lamp, mirrors) must be returned to the start-of-scan position after each scan operation. This return time reduces throughput and leaves an inter-document gap which ordinarily must be discharged in some way. By forming a complete document image at the photoreceptor, at greater than process velocity, a time interval (precess interval) is established during which the system scan elements can be returned to the start-of-scan position before the next image from on the photoreceptor advances to the next start-of-scan position. Thus, throughput could be increased or, alternatively, the process speed can be set at a lower speed for equivalent copy rate. Scan return velocities for a given copy rate can also be minimized.
The prior art precession scan systems have heretofore been limited in the output precessing speeds to about 75 cpm. According to one aspect of the present invention, there is provided a precession scanning system capable of process speeds up to 110 cpm. There is also provided an improved drive system for the scan components capable of changing precession scan speeds and precession ratio in accordance with different document sizes and magnification ratios. More particularly, the invention is directed to a precession scanning system for an electrophotographing printing machine comprising:
an object side scanning system adapted to incrementally scan/illuminate a document lying on a platen surface at a scan speed V.sub.1 ;
a lens positioned along the optical path, said object side scanning system reflecting said incrementally scanned images into said lens;
an image side scanning system adapted to reflect the projected incremental document images onto the surface of a photoreceptor in a direction opposite the direction of the moving photoreceptor along a precession distance and at a precession scan speed V.sub.3 ; and
means for driving said object and image side scanning system in a scan and rescan direction at speeds relative to each other and to the photoreceptor speed so as to enable the image to be precessed onto the photoreceptor.